1. Field of the Invention
The present invention relates generally to aluminum base alloys and more particularly, Al—Zn—Cu—Mg aluminum base alloys.
2. Description of Related Art
Al—Zn—Cu—Mg aluminum base alloys have been used extensively in the aerospace industry for many years. With the evolution of airplane structures and efforts directed towards the goal of reducing both weight and cost, an optimum compromise between properties such as strength, toughness and corrosion resistance is continuously sought. Also, process improvement in casting, rolling and annealing can advantageously provide further control in the composition diagram of an alloy.
Thick rolled, forged or extruded products made of Al—Zn—Cu—Mg aluminum base alloys are used in particular to produce integrally machined high strength structural parts for the aeronautic industry, for example wing elements such as wing spars and the like, which are typically machined from thick wrought sections.
The performance values obtained for various properties such as static mechanical strength, fracture toughness, resistance to stress corrosion cracking, quench sensitivity, fatigue resistance, level of residual stress will determine the overall performance of the product, the ability for a structural designer to use it advantageously, as well as the ease it can be used in further processing steps such as, for example, machining.
Among the above listed properties some are often conflicting in nature and a compromise generally has to be found. Conflicting properties are, for example, static mechanical strength verses toughness and strength verses resistance to stress corrosion cracking.
Al—Zn—Mg—Cu alloys with high fracture toughness and high mechanical strength are described in the prior art.
As an example, U.S. Pat. No. 5,865,911 describes an aluminum alloy consisting essentially of (in weight %) about 5.9 to 6.7% zinc, 1.8 to 2.4% copper, 1.6 to 1.86% magnesium, 0.08 to 0.15% zirconium balance aluminum and incidental elements and impurities. The '911 patent particularly mentions the compromise between static mechanical strength and toughness.
U.S. Pat. No. 6,027,582 describes a rolled, forged or extruded Al—Zn—Mg—Cu aluminum base alloy products greater than 60 mm thick with a composition of (in weight %), Zn: 5.7-8.7, Mg: 1.7-2.5, Cu: 1.2-2.2, Fe: 0.07-0.14, Zr: 0.05-0.15 with Cu+Mg<4.1 and Mg>Cu. The '582 patent also describes improvements in quench sensitivity.
U.S. Pat. No. 6,972,110 teaches an alloy, which contains preferably (in weight %) Zn: 7-9.5, Mg: 1.3-1.68 and Cu 1.3-1.9 and encourages keeping Mg≦(Cu+0.3). The '110 patent discloses using a three step aging treatment in order to improve resistance to stress corrosion cracking. A three step aging is long and difficult to master and it would be desirable to obtain high corrosion resistance without necessarily requiring such a thermal treatment.